1. Field of the Invention
The present invention relates to a method and device for inspecting a wafer surface and, more specifically, relates to an inspection method and device which distinguishes between extraneous substances sticking on a surface of a silicon wafer and crystal defects existing on the surface thereof.
2. Background Art
A silicon wafer base material for semiconductor ICs is manufactured from a highly purified polycrystalline silicon. Namely, the silicon wafer is manufactured in the following manners in that at first an ingot of a moncrystalline silicon is prepared via a pull-up method, for example the prepared ingot is sliced into thin plates, the surface of the sliced thin plate is polished to a mirror surface, and thereafter extraneous substances sticking on the surface thereof are carefully cleaned off. In spite of such careful cleaning, some sticking extraneous substances may remain on the surface thereof. If many sticking extraneous substances remain on the wafer surface, the quality of ICs produced from the silicon wafer is degraded. For this reason, the surface of the silicon wafer is inspected for remaining sticking extraneous substances and the degree thereof via a surface inspection device.
FIG. 10(a) shows a constitution of a conventional wafer surface inspection device. As illustrated in FIG. 10(a), a the surface inspection device is constituted by, for example a rotatable and movable table 2, an inspection optical system 3 and a data processing unit 4. A silicon wafer (hereinbelow simply called a wafer) 1, which is an object for the inspection, is placed on the rotatable and movable table 2. The inspection optical system arranged above the wafer 1 includes a laser source 31 provided with a laser oscillating tube. The laser beam LT outputted from the laser source 31 is made parallel via a collimator lens 32 and is scanned in the X direction by a vibrating mirror 33. Then, the scanned laser beam is focused by a focusing lens 34 as a laser spot Sp (hereinbelow simply called a spot Sp) and is projected perpendicularly onto the surface of the wafer 1 to scan the wafer in response to the movement of the wafer 1.
The wafer 1 is rotated by the rotatable and movable table 2 as well as moved in the radial direction (X direction). Thereby, the spot Sp scans the surface of the wafer 1 in a spiral manner so that the entire surface of the wafer 1 is scanned. The driving of the rotatable and movable table 2 is controlled via the data processing unit 4 which is explained later.
When an extraneous substance e exists on the surface of the wafer 1 as illustrated in FIG. 10(b), the spot Sp induces scattering light Se in random directions due to the extraneous substance e. A part of the scattering light Se is condensed by a condensing lens 35, the optical axis of which forms a 45.degree. angle with respect to the wafer surface, and is received by a photomultiplier tube (PMT) 36 serving as a photoelectric converter. The incident light into the PMT 36 is converted herein into an electrical signal and the converted received light signal is inputted into an extraneous substance detection circuit 41.
The extraneous substance detection circuit 41 compares the received light signal with a predetermined threshold value V.sub.TH via a differential amplifier of one side amplification, and amplifies the component exceeding the threshold value V.sub.TH. Thereby, a detection signal (analog signal) of which the noise component is removed is input into the data processing unit 4.
The detection signal is converted into a digital value via an A/D converter circuit (A/D) 42b provided in a data processing device 42 and is stored once in a memory 42c via an MPU 42a when the MPU 42a executes a predetermined programs the detection data is processed together with the data of the concerned scanning position (detection position). As a result, the size of the extraneous substance e is judged depending on the detection values Further, the number of the extraneous substances is counted. Still furthers through execution of a predetermined program via the MPU 42a, data of extraneous substances representing the size, number and position of the extraneous substances e are produced and outputted such as to a printer 43 and a display (not shown) to display the conditions of the extraneous substances in a map form. Still furthers the A/D 42b can be provided outside the data processing device 42.
The above mentioned spot Sp has a diameter of about a few .mu.m and is a very intense light, and a condenser lens having a large diameter and a wide condensing angle is used for the condenser lens 35. Furthers the PMT 36 has a large amplification rate and a low noise characteristic. Thereby, even an extraneous substance or a defect having a diameter of about 0.1 .mu.m can be detected.
Further, the scanning by the spot Sp for the wafer 1 can be performed by XY scanning instead of the rotating scanning method.
In connection with a recent improvement of the integration density of ICs and related miniaturization of their wirings, the limit of allowable size of extraneous substances to be detected is further lowered. For this reason, even a defect due to loss of atoms in the silicon atom lattice of the wafer 1 is considered to be a problem which is explained with reference to FIGS. 11 and 12.
The wafer is constituted by a single crystal in which a multiplicity of silicon atoms Si are mutually connected in a lattice form. As illustrated in FIG. 11, when silicon atoms Si are oxidized and microscopic oxides thereof are formed on the surface thereof, and if the microscopic oxides are removed by cleaning, a loss of atoms can be induced which causes a crystal defect. Such a crystal defect is called a Crystal Originated Particle (COP), which term is used hereinbelow.
In FIG. 11, a plurality of contigous silicon atoms Si (in the drawing three) are lost. When observing the portion of the water loss via a microscopes the cross section thereof is recessed as illustrated in FIG. 12. Although a variety of diameters .phi. and the depths .delta.z of such recesses can be observed, such recesses show a common characteristic that the depth .delta.z is comparatively small in comparison with the diameter .phi. thereof. For example, where a diameter .phi. is 1.about.2 .mu.m, the depth .delta.z thereof is about 0.05.about.0.1 .mu.m which is about one twentieth of the diameter. The size of a COP defect varies depending on ICs, in that a COP having diameter .phi. of 2 .mu.m is no problem for an IC memory of 16M bits, but is a problem for an IC memory of more than 64M bits.
The number of COPs varies depending on the pull-up speed when manufacturing an ingot of a single crystal silicon and number of cleanings of the wafer. On the other hand, the number of sticking extraneous substances decreases depending on the number of cleanings. Therefore, the pull-up speed and the number of cleanings have to be determined properly. For this reasons it becomes necessary to separately measure the number and size of COPs and sticking extraneous substances.
Therefore, it is demanded to measure the COPs with a wafer surface inspection device and to obtain an evaluation data thereof. However, with the above explained detection optical system 3, sticking extraneous substances as well as COPs are detected at the same time such that separate measurement of the number of both or the number and size of both has not been possible.
With regard to the extraneous substance inspection device, U.S. Pat. No. 5,245,403 entitled "APPARATUS FOR DETECTING EXTRANEOUS SUBSTANCES ON A GLASS PLATE" of the same assignee as the present application discloses two light projecting systems of high and low angles.